In recent years, energy use has become more and more efficient, and demands are increasingly being made, mainly from transformer manufacturers and the like, for an electrical steel sheet with high flux density and low iron loss.
The flux density can be improved by accumulating crystal orientations of the electrical steel sheet in the Goss orientation. JP4123679B2, for example, discloses a method of manufacturing a grain-oriented electrical steel sheet having a flux density B8 exceeding 1.97 T.
With regards to iron loss, measures have been devised from the perspectives of increasing purity of the material, high orientation, reduced sheet thickness, addition of Si and Al, magnetic domain refining, and the like (for example, see “Recent progress in soft magnetic steels”, 155th/156th Nishiyama Memorial Technical Seminar, The Iron and Steel Institute of Japan, Feb. 1, 1995). In a high flux density material in which B8 exceeds 1.9 T, however, iron loss properties tend to worsen as the flux density is higher, in general. The reason is that when the crystal orientations are aligned, the magnetostatic energy decreases and, therefore, the magnetic domain width widens, causing eddy current loss to rise. To address this issue, one method of reducing the eddy current loss is to apply magnetic domain refining by enhancing the film tension or introducing thermal strain. Generally, film tension is applied using the difference in thermal expansion between the film and the steel substrate, by forming a film on a steel sheet that has expanded at a high temperature and then cooling the steel sheet to room temperature. Techniques to increase the tension effect without changing the film material, however, are reaching saturation. On the other hand, with the method of improving film tension disclosed in Ichijima et al., IEEE TRANSACTIONS ON MAGNETICS, Vol. MAG-20, No.5 (1984), p. 1558, FIG. 4, the strain is applied near the elastic region, and tension only acts on the surface layer of the steel substrate, leading to the problem of a small iron loss reduction effect.
Possible methods of introducing thermal strain include using a laser, an electron beam, or a plasma jet. All of these are known to achieve an extremely strong improvement effect in iron loss due to irradiation.
For example, JP7-65106B2 discloses a method of manufacturing an electrical steel sheet having iron loss W17/50 of below 0.8 W/kg due to electron beam irradiation. Furthermore, JP3-13293B2 discloses a method of reducing iron loss by applying laser irradiation to an electrical steel sheet.
When using a laser, electron beam, or plasma jet to introduce thermal strain under conditions that greatly improve iron loss properties, however, the film on the irradiation surface may in some cases rupture, exposing the steel substrate and leading to a remarkable degradation in the corrosion resistance of the steel sheet after irradiation. A method that introduces thermal strain with a plasma jet to not impair the corrosion resistance is known (see JP62-96617A). However, that method requires that the distance between the plasma nozzle and the irradiation surface be controlled in μm increments, causing a considerable loss of operability.
In the case of a laser, techniques exist to suppress damage to the film due to irradiation by lowering the laser power density through a change in the beam shape, as disclosed in JP2002-12918A and JP10-298654A. Even if the laser is widened in the irradiation direction to increase the irradiation area, however, heat near the irradiated portion does not spread sufficiently when the irradiation speed is high, but rather accumulates, which raises the temperature and ends up damaging the film. Furthermore, when attempting to achieve an iron loss reduction effect equal to or greater than the values disclosed in JP '918 or JP '654 (such as 15% or more) with a laser, irradiation at a higher output becomes necessary, making it impossible to avoid damage to the film.
As a method of preventing degradation of corrosion resistance when applying laser irradiation to the steel sheet surface, the irradiated surface may be recoated after irradiation to guarantee corrosion resistance. Recoating after irradiation, however, not only increases the cost of the product, but also presents the problems of increased sheet thickness and a decreased stacking factor upon use as an iron core.
By contrast, when irradiating with an electron beam, JP5-311241A and JP6-2042A, respectively, disclose methods of suppressing damage to the film due to irradiation by configuring the irradiation beam in sheet form (JP '241) and by using a beam with a single stage diaphragm and forming the filament shape as a ribbon (JP '042). Furthermore, JP2-277780A discloses achieving a steel sheet with no damage to the film by press fitting a film to a steel substrate with a high acceleration voltage, low current electron beam.
With the method to configure the electron beam in sheet form, however, output at the inner portion of the sheet-form irradiation surface is not uniform, leading to problems such as troublesome adjustment of the optical system. Also, under electron beam irradiation conditions for which iron loss decreases further, it was revealed that damage to the film due to irradiation occurs when forming the filament in a ribbon shape or adopting a single stage diaphragm. Furthermore, the method disclosed in JP '780 not only requires strain removal annealing after electron beam irradiation but also cannot be said to achieve a sufficient iron loss reduction effect.
It could therefore be helpful to provide a grain-oriented electrical steel sheet suitable for use as an iron core of a transformer or the like and having low iron loss without deterioration of corrosion resistance, as well as a method of manufacturing the grain-oriented electrical steel sheet.